Heat oven to 350°F. Line 2 baking sheets with parchment paper and lightly coat with cooking spray.
In a medium bowl, whisk together sugar, cocoa powder, and salt. Using an electric mixer, beat together eggs and vanilla. Add sugar mixture and mix to combine; fold in walnuts and chocolate chips.
Spoon batter (about 1 ½ Tbsp per cookie) onto prepared baking sheets, spacing 2 in. apart, and sprinkle with flaky sea salt.
Bake, rotating positions of pans once, until cookies are puffed and tops begin to crack, 12 to 14 minutes. Let cool on baking sheets 5 minutes, then slide parchment and cookies to wire racks to cool completely.
